I have a E90 BMW M Sport with alcantara interior. The interior is manual but has electric bolsters and have split fold down rear seats.

I am looking to replace the interior with leather interior and wanted to know can I change the interior with non fold down rear seats or are the rear seat fittings different therefore are not straight forward to change over.

You will not have any problems fitting non fold down seats. I swapped interiors between two cars, one with fold down and one with standard. Removing and refitting the fold down seats was quite fiddly and time consuming, but the standard seats are easy peasy.